Fibrin formation is an irreversible process; as a result, the structure of fibrin is determined by the kinetics of a self-assembly process. We shall use light scattering in order to follow the formation of fibers and to measure fiber thickness, electron microscopy to determine the size and shape of fibers at different stages of the assembly and to study the frequency and structure of network branch-points, elasticity measurements to detect and follow the formation of the network of fibers. We shall investigate methods for preparing fibrin in a form suitable for x-ray diffraction studies. We shall devote a major part of our effort to study physiologically important interactions of fibrin with other plasma components using these physical techniques, following at the same time changes in the chemistry of the fibrin with biochemical analytical techniques. For example, we shall use SDS gel electrophoresis to detect changes in the covalent structure due to chain scission or crosslinking. Blood components whose interactions with fibrin we shall study include: plasmin, factor XIII, fibrin degradation products, calcium ion, and thrombosthenin. We shall also study the effects of ancrod, reptilase and crotalase (thrombin-like enzymes from snake venoms) and of ethanol and dextran on the formation of fibrin fibers.